The complexity of tissue structure presents challenges in creating physiologically relevant in vitro models that closely mimic in vivo cell microenvironments. One technique used to model tissue mimetic biological interfaces is membrane inserts (such as Millicell™, Transwell™) that facilitate co-culture of cells on opposite sides of the barrier. However, many physiologically realistic in vitro models require dynamic flow conditions to achieve comparable cell microenvironments.
Microfluidic-based cell culture systems can overcome certain limitations in perfusion, and these systems can be well-suited for multiplexed in vitro models. See, for example, Wu et al. in Proc. Natl. Acad. Sci. USA, 2006, 103, 2480. Several microfluidic approaches that utilize either horizontally integrated membrane layers or vertically defined features to create a biological barrier have been described. See, for example, Carraro et al. in Biomed. Microdevices, 2008, 10, 795; Duncanson et al. in TERMIS-NA 2008 Conference & Expo, San Diego, Calif., 2008; Ma et al. in Lab Chip, 2005, 5, 74; Huh et al. in World Congress on Medical Physics and Biomedical Engineering 2006, 2007, 258; Huh et al. in Science, 2010, 328, 1662; Shao et al. in Biomed. Microdevices, 2010, 12, 81; Jang et al. in Lab Chip, 2010, 10, 36; Lee et al. in Biotechnol. Bioeng., 2007, 97, 1340; and Zanell et al. in Trends Biotechnol., 2010, 28, 237. Such microfluidic cellular constructs have been seeded with different cell types on opposite sides of the membrane to demonstrate models of an alveolar-capillary interface, smooth muscle cell-endothelial interface, and endothelial cell-astrocytic end feet interface.
Although microfluidic devices reported in the literature can be useful for investigating biological interfaces, their fabrication and assembly presents multiple challenges. These challenges manifest as a trade-off between geometric and mechanical complexity (e.g., integrated membranes, scaffolds, and multilayer formats) and the ability to visualize cells using high-resolution microscopy. Technical advances in high-content screening have enabled the practical implementation of high-throughput sub-cellular, high-resolution imaging, but these data are not available when complex culture systems are optically inaccessible. See, for example, Wlodkowic et al. in Anal Chem, 2009, 81, 9828.
Accordingly, the need exists for new microfluidic devices that are amenable for use with imaging equipment and can present cells in an environment that adequately mimics in vivo conditions. The present invention addresses the need for improved microfluidic devices and methods for imaging cells and provides other related advantages.